"This decision was not taken lightly, and reflects U.S. concerns with mounting arrears at UNESCO, the need for fundamental reform in the organization, and continuing anti-Israel bias at UNESCO," State Department Spokesperson Heather Nauert said.

The U.S. indicated to UNESCO Director-General Irina Bokova the desire to remain engaged with the organization as a non-member observer state "to contribute U.S. views, perspectives and expertise," she said.

The withdrawal will take effect on Dec. 31, 2018, and the U.S. will continue to remain a full member of UNESCO until that time.

UNESCO is best known for its World Heritage program to protect cultural sites and traditions around the world. The agency also works to improve education for girls in desperately poor countries and in scientific fields, to promote better understanding of the horrors of the Holocaust and to defend media freedom, among other activities.

While the U.S. stopped funding UNESCO after it voted to include Palestine as a member in 2011, the State Department has maintained a UNESCO office at its Paris headquarters and sought to weigh in on policy behind the scenes.

The U.S. previously pulled out of UNESCO in the 1980s because Washington viewed it as mismanaged and used for political reasons, then rejoined it in 2003.

The decision by the Trump administration comes as the U.N. Educational, Scientific and Cultural Organization is voting to choose a new director this week, in tense balloting overshadowed by the agency's funding troubles and divisions over Palestinian membership.

Many saw the vote to include Palestine as evidence of long-running, ingrained anti-Israel bias within the United Nations, where Israel and its allies are far outnumbered by Arab countries and their supporters.

The Trump administration has been preparing for a likely withdrawal for months, and a decision was expected before the end of the year, U.S. officials previously told the Associated Press. Several diplomats who were to have been posted to the mission this summer were told that their positions were on hold and advised to seek other jobs.

In addition, the Trump administration's proposed budget for the next fiscal year contained no provision for the possibility that UNESCO funding restrictions might be lifted.